In aircraft, increasing use is being made of central liquid cooling systems. A typical liquid cooling system comprises a refrigerating machine that delivers cold liquid coolant. The liquid coolant is conveyed by means of a line network to one or more heat loads in the aircraft. A plurality of heat loads connected in series and/or in parallel may be supplied with liquid coolant. The liquid coolant, after cooling the heat load(s), is returned to the refrigerating machine so that it circulates permanently in a closed circuit. Liquid cooling systems have the advantage that the lines for transporting the liquid coolant may be of a much thinner design than in a cooling system that uses a is gaseous fluid, for example air, as a coolant because a liquid coolant is able to absorb much more heat than a gaseous coolant. The thinner lines have the advantage that they are easier to install in an aircraft and more space is available for other components of the aircraft. With a liquid coolant, moreover, greater distances between the refrigerating machine and the heat load may be bridged. The liquid coolant may be for example water, a mixture of water and glycol, or a refrigerant medium liquid based on fluorocarbons. The liquid coolant may have a temperature between ca. −9° C. and ca. 10° C. or ca. 1° C. and ca. 10° C. It is also conceivable for the coolant to have a lower or higher temperature.
The liquid cooling system may cool a food trolley as well as food and drinks in the galley. For this purpose, liquid-gas heat exchangers are provided. The liquid coolant passes into the liquid-gas heat exchanger and cools air that is directed by means of a fan into the food trolley and onto the food and drinks in the galley. It is also possible to cool the flight control computers (avionics bay) or the entertainment systems (IFE: in-flight entertainment) with liquid coolant. The liquid cooling system may also be used to individually cool cabin areas, for example first-class suites or the area around a business-class seat. The refrigerating machine may be a compression-type refrigerating machine that is disposed inside the pressurized cabin. This has the drawback that the waste heat of the compression-type refrigerating machine additionally loads the air conditioning system of the cabin because the air conditioning system has to cool an additional heat load in the pressurized cabin. It is also possible for the refrigerating machine to take the form of a compression-type refrigerating machine that is disposed outside of the pressurized fuselage. In this case, the waste heat of the refrigerating machine is released into the environment.
It is self-evident that such a liquid cooling system has to be designed for the least favourable load scenario. One of the least favourable load scenarios is a high ambient temperature when the aircraft is on the ground. In this situation, a compression-type refrigerating machine arranged outside of the pressurized fuselage has to provide considerably more cooling capacity than during a flight at high altitude, when a lower outside temperature prevails. In order to provide the cooling capacity needed on the ground, a compressor, an evaporator and a condenser of the compression-type refrigerating machine have to be of a powerful design. This is however linked to an increased weight. This means that the aircraft while in flight has to carry extra weight in the form of the powerful compressor, evaporator and condenser in order to provide the required cooling capacity while on the ground. This powerful design of the components is not needed in flight. The extra weight entailed by the powerful design leads to higher fuel consumption and reduces the available payload. This compression-type refrigerating machine is moreover more difficult to install in an aircraft as the powerful compressor, evaporator and condenser take up extra installation space.